In conventional chairs used for business, etc., a seat plate that is a frame part of a seat body is supported on an upper end of a leg assembly via a seat receiving member such as a slide frame (e.g., see Patent Documents 1 to 3).
In the chairs described in Patent Documents 1 to 3, engaging grooves that extend in a forward/backward direction and have approximately U-shaped cross sections that are open upward or downward, and protrusion that are engaged with the engaging groove are provided between the seat receiving member and the seat plate. Also, upward/downward displacement of the seat plate relative to the seat receiving member is regulated by a separate displacement regulating means (fall prevention means).
In the case of these chairs, since the engaging grooves and the protrusions that are engaged with each other are provided with the displacement regulating means acting as a separate member, the number of components is increased, and a structure easily becomes complicated. For this reason, chairs improving these drawbacks have currently been devised (e.g., see Patent Documents 4 and 5).
In the chairs described in Patent Documents 4 and 5, engaging grooves that extend in a forward/backward direction and have approximately U-shaped cross sections that are open to an inner side in a width direction are provided at both left and right edges of a seat plate, and flange parts respectively engaged with the left and right engaging grooves of the seat plate are provided at both left and right edges of a seat receiving member. Upward/downward displacement (falling off) of the seat plate relative to the seat receiving member is hindered by the engagement of the flange parts with the engaging grooves. In these chairs, the engaging grooves of the seat plate are provided at almost part of the seat plate in the forward/backward direction, and the upward/downward displacement of the seat plate is regulated at almost part of the seat plate in the forward/backward direction.
Also, in the chair described in Patent Document 5, from the viewpoint of moldability and sitting comfort, the seat plate is formed of a synthetic resin having flexibility.
On the other hand, a chair baseplate (seat plate) is disclosed in Patent Document 6, which has a frame part surrounding the periphery of a pressure receiving part and numerous protrusions defining numerous meshes at the pressure receiving part and in which the meshes are open.
A chair baseplate (backrest) is disclosed in Patent Documents 7 and 8, in which slit-like openings are linearly arranged in a longitudinal direction and are arranged in a zigzag form in a width direction by shifting a position of the longitudinal direction.
A chair baseplate (backrest) is disclosed in Patent Document 9, in which numerous rhombic openings that extend vertically are arranged vertically and horizontally.
On the other hand, as a chair in which a seat body including a seat plate and a seating body such as a cushion is supported on a seat receiving structure supported on a leg body to be movable backward and forward, for example, the chairs disclosed in Patent Documents 1 to 5 and 10 described above are known.
In the chairs of this field, both left and right lateral portions of the seat plate are supported to be slidable in a forward/backward direction at both left and right lateral portions of the seat receiving structure supported on the leg body. This structure is intended to provide a comfortable chair in which, as a position of a front end of the seat plate can be variously adjusted according to a size of the body of an user, a back of a knee of the user is properly supported from a lower side by the seat body, and the user does not get tired even when seated for a long time.